1. Field of the Invention
The present invention relates to an activated alumina formed body. More specifically, the present invention relates to an activated alumina formed body having a high bulk density and a large macro-pore volume, and a method for producing the activated alumina formed body.
2. Related Art of the Invention
Transition alumina formed bodies, in which alumina has a variety of crystal phases other than α form and is represented by Al2O3, are used as drying agents, adsorbents, catalysts, various carriers and the like. For these usages, it is preferred that an alumina formed body to be used have a large pore volume in order to increase the diffusion rate of molecules into the alumina formed bodies. Specifically, it is preferred that the alumina formed bodies have a large volume (macro-pore volume) of macro pores having a radius of about 0.1 μm or more, more preferably a radius of about 0.3 μm or more.
On the other hand, when an activated alumina formed body is used in a fixed bed facility as a catalyst carrier, an adsorbent or the like, the activated alumina formed body is preferably packed in a large amount in order to improve catalyst life, the adsorption amount therewith or the like and, therefore, the activated alumina formed body having a high bulk density is desired.
The following techniques are known as methods for producing an activated alumina formed body having a large macro-pore volume.
Method (1): An aluminum salt is neutralized to obtain an alumina gel, while controlling the precipitation condition of alumina, followed by being washed, dried, formed and calcined to obtain an activated alumina formed body (for example, as in Japanese Patent Publication No. 2-1767, which corresponds to U.S. Pat. No. 4,555,394).
Method (2): A rehydratable alumina is mixed with a fibrous flammable organic pore-generating agent, followed by being formed, rehydrated and calcined at a temperature of 450° C. to 650° C., to obtain an activated alumina formed body (for example, as in Japanese Patent Application Laid-Open No. 49-6006, which corresponds to Great Britain Laid-Open Patent Publication No. 1,356,239).
Method (3): An organic pore-generating agent such as Polyethylene glycol, polyvinyl alcohol, crystalline cellulose and carbon black is utilized to obtain an activated alumina formed body. And a polymethacrylic acid ester is utilized as an organic pore-generating agent, which is removed at a low temperature, to obtain an activated alumina formed body (for example, as in Japanese Patent Application Laid-Open No. 8-245281).
However, by any of the methods (1)-(3), the total pore volume increases with increase in macro-pore volume, which results in reduction in bulk density. As a result, it is difficult to obtain an activated alumina formed body having macro-pores with a high bulk density. In addition, by any of the above methods, the resultant activated alumina formed body costs high.
On the other hand, the following method is known as a method for producing an alumina formed body at a low cost.
Method (4): Bayer process is conducted using a gibbsite-phase aluminum hydroxide having a median particle size of about 1 μm to 35 μm and having a quartile deviation of a particle size distribution of about 1.5 or less, to obtain an rehydratable alumina. The rehydratable alumina is formed into a shape of a formed body, followed by being rehydrated and then being calcined, to obtain an activated alumina formed body (for example, as in Japanese Patent Publication No. 63-24932, which corresponds to U.S. Pat. No. 4,444,899). The obtained activated alumina formed body tends to have a large macro-pore volume.
However, method (4) also has problems such that the activated alumina formed body has a large total pore volume, while having a large macro-pore volume. Specifically, as shown in Example 1 of the specification of the above ) publication JP-A-63-24932, the resultant activated alumina body has a packed bulk density of as low as 0.58 kg/l (=g/cm3), since the total pore volume increases. It is difficult in method (4) to provide an activated alumina formed body having macro pores and yet having a high bulk density.